Japanese Patent Laid-Open Publication No. 63-298128 discloses a pressure sensor including an insulating layer formed on a metal substrate and a thick-film resistor formed on a surface of the insulating layer. The insulating layer glass-glazed used for this pressure sensor employs glass having a thermal expansion coefficient close to that of metallic material.
Japanese Patent Laid-Open Publication No. 61-67901 discloses that a thermal expansion coefficient of glass used for a resistor element is made equal to that of a substrate so as to cause temperature coefficient of resistance (TCR) characteristics of a glazed stainless-steel substrate to match those of a strain-sensitive resistor element formed on the substrate. The substrate, a base, has a thermal expansion coefficient of 70×10−7/° C., and the resistor element is made of material having a thermal expansion coefficient 7×10−7/° C. In this case, a commercially-available material for the element (having the thermal expansion coefficient of about 70×10−7/° C.) for alumina substrates can be used.
Japanese Patent Laid-Open Publication No. 6-294693 discloses that a thermal expansion coefficient of glass frit contained in a strain-sensitive resistor element is made close to that of a substrate.
Japanese Patent Laid-Open Publication No. 9-273968 discloses a dynamic-quantity sensor that prevents mutual diffusion between a strain-sensitive resistor element and glass of a base for stabilizing characteristics of the resistor element. In this sensor, two kinds of resistor elements improve matching between the resistor elements and the glass of the base. The sensor includes a first resistor element on an insulating layer on a metal substrate, and a second resistor element on the first resistor element. A resistance of the first resistor element is determined to be larger than that of the second resistor element, thereby reducing an influence to a resistance of the entire sensor even if the first resistor element is influenced by the insulating layer.
Japanese Patent No. 3010166 discloses a glass layer containing particulate alumina and particulate zinc oxide and provided between a glass layer and a strain-sensitive resistor element so as to reduce influence of mutual diffusion of the glass layer and the resistor element formed on a metal substrate.
As various types of devices using a strain-sensitive resistor element are used, a gauge factor (GF), a rate of a change in a resistance per a unit amount of strain of a resistor element, is demanded to improve. However, the resistor element has its characteristic more unstable according to an increase of the GF.
For instance, if the base substrate has a thermal expansion coefficient of 100×10−7/° C., a resistor element needs to be made of material having a thermal expansion coefficient of about 100×10−7/° C. Similarly to this, if a substrate having a thermal expansion coefficient of 140×10−7/° C., a resistor element needs to be made of material having that of 140×10−7/° C. However, commercially-available material of resistor element is for alumina substrates. That is, material having a thermal expansion coefficient other than that of the material for the alumina substrates is not available in the market, and it is difficult to newly develop such material. This is because material of resistor elements needs to optimize not only for the GF but for various parameters, such as the TCR, noise characteristic, and reliability. Thus, respective materials corresponding to various substrates having respective thermal expansion coefficients cannot substantially be developed.
The following problems still exist even if the influence by mutual diffusion between material of a strain-sensitive resistor element and material of a base substrate is reduced. Since the difference between respective thermal expansion coefficients of a metal substrate and a resistor element can not be eliminated, plural types of resistor paste corresponding to the thermal expansion coefficients of substrates need to be prepared. Further, a resistor element cannot avoid to receive a stress inside thereof due to the difference of the thermal expansion coefficients of a metal substrate and material of the resistor element.
Further, if a substrate of an actual load sensor is made of metal, the resistance of a strain-sensitive resistor element is influences by factors other than the thermal expansion coefficient. For example, in the case that a load sensor having complicated dimensions and shape according to request by a user is manufactured by die-cutting a thick metal plate with a mold, an internal stress of the metal plate causes a problem. In this way, the above-mentioned parameters, such as residual stress occurring when processing a substrate with stamping die or the like, correction of warpage occurring when die-cutting (generally, corrected by warping it inversely), and annealing, influence the thermal expansion coefficient of the actual substrate Therefore, substrates made of metallic material having a thermal expansion coefficient of 100×10−7/° C. and having thicknesses of 1 mm, 2 mm, and 5 mm, respectively, change in the thermal expansion coefficient and the amount of warpage. For example, the substrates formed by die-cutting metal plates having thicknesses of 1 mm and 2 mm with the same mold, respectively, the substrates exhibit different warpages immediately after the die-cutting. Accordingly, even if warpage is corrected, respective amounts of warpage of the substrates are slightly different after the substrates are fired at 850° C. A thick substrate having a thickness of 5 mm, for example, has warpage much different from those of the above this substrates after the firing since a die-cutting method applied to the thick substrate is different than the thin substrates. Such warpage and deformation of a substrate influence a resistor element like the thermal expansion coefficient, thus tending to make the resistance unstable. Further, such distortion occurs during laser processing as well as the above-mentioned die-cutting.
In the conventional load sensor, various stresses are produced in a resistor element on a substrate due to the difference of substrates (e.g. material, thickness, shape), and processing methods (e.g. residual stress in machining, annealing, die-cutting, and stamping). The resistance tends to change over time according to an increase of the GF.